If they do not make any adjustment to the team by introducing some new signings outside of the French League following the exit of Mbappe, trust me if I tell you that their highest achievement will be the French Lig 1 title next season. They should never nurse any ambition of getting close to the semi finals of the champions league like they did this last season. I am not saying that they do not have good players, just that the squad is not just competitive enough to face other tough European teams like Bayern Munic, Man City or Real Madrid. You should not forget that it is expected that Mbappe leaving will create a huge vacuum in the team which require an external person to fill.
PSG's strength will be clearly different next season as a result of Mbappe's departure to Real Madrid, but to just become champions again in Ligue1 I also think that it won't be that difficult for PSG because considering that the other teams in Ligue1 are also not experiencing such significant developments. However, for the UCL competition, PSG certainly won't be able to do much if there are no more reliable players in their squad next season. Moreover, other teams that will also enter the UCL next season, such as Real Madrid, already have quite an extraordinary line-up of players next season.